Numbering Plans

A numbering plan describes the endpoint addressing used in a voice system and is analogous to the IP addressing used in a data network. Unlike a data network, today the end user must know and enter the endpoint address to communicate with another endpoint. Because the end user must know and enter the address of the endpoint he wants to communicate with, ease of use is a major focus of numbering plans.

Numbering plans are classified as private or public. A private numbering plan is used within a single organization, and the organization defines and administers it. A public, or PSTN, numbering plan is defined for a single country or group of countries. Government agencies or service providers typically administer PSTN numbering plans, subject to governmental oversight.

Private Numbering Plans

When you are designing a private numbering plan, you must consider numerous factors. The most important factor is the number of devices requiring a number. This includes devices such as end-user phones, common area phones, fax machines, modems, and security systems. You should select an extension length that accommodates the existing number of devices and allows for growth. If you have 700 devices, you would probably want to use four-digit extensions, which allow for 10,000 numbers, even though you could assign a three-digit extension, allowing for 1000 numbers, to all existing devices. Increasing the extension length of an existing system to accommodate additional devices requires substantial planning to ensure that all devices are reachable during the changeover period. It also requires that end users are informed and possibly retrained.

Another important factor is how many locations you have. Consider the following two companies. Company A has a single location with 4000 devices. Company B has 200 locations, each with 20 devices. Both companies have to address 4000 devices, but Company B has to consider more factors than Company A. Calling patterns, PSTN connections, and alternate routing requirements impact the Company B numbering plan.

Private Numbering Plan Design Considerations

When you are designing a private numbering plan, you need to address the following considerations:

Number of addressable devices Most organizations try to use the fewest possible digits for internal calling to make it easy for users to remember and dial extensions. To allow for growth, use at least one more digit than the minimum required. For example, if you have 7,000 addressable stations, it is possible to use a four-digit numbering plan, but a five-digit numbering plan, which allows for 100,000 numbers, supports significantly more growth.

Number of locations Organizations typically use fewer digits for calls within a location, and site codes plus extensions for calls between locations. The number of digits in the site code must support the number of sites.

Methods for assigning site codes Some organizations use a mnemonic, assigning site codes based on the location. For example, New York City (NYC) would be assigned a site code of 692, and Tampa (TPA) would be assigned a site code of 872. Other organizations use existing numeric codes assigned to branch offices.

Inbound call routing The method that is used to accept calls from the PSTN determines how numbers are assigned to devices.

Service providers offer direct inward dial (DID or DDI) services to provide a range of PSTN numbers. Organizations can pay for a sequential block of these DIDs and allow external callers to place calls directly to the intended recipients. In this case, organizations use the DID number to assign the extension.

Many organizations opt to publish a single number for external callers. This number is routed to an auto attendant (AA) or receptionist who forwards the call to the intended recipient. This option is less expensive than DID services and allows more flexibility in assigning extensions.

Some organizations use a combination of DID and AAs. They might assign DIDs to employees who frequently receive outside calls, such as salespeople. Calls to all other employees are routed through the AA.

PSTN access codes A PSTN access code distinguishes internal calls from external calls. This eliminates interdigit timeout when an internal numbering plan uses fewer digits than a PSTN numbering plan. It is customary to use 9 as a PSTN access code in the United States. In many other countries, 8 is used as the PSTN access code.

One effect of using a PSTN access code is that you cannot use extensions beginning with this number. Consider a company using a four-digit numbering plan and a PSTN access code of 9. The company has purchased a 10,000 DID range from the service provider of 813 555-0000 thru 813 555-9999. If the company assigns extensions in the 9XXX range, the call processing system will not be able to distinguish internal calls from external calls. When a call is placed to extension 9500, the system will have to wait to see if additional digits will be dialed to determine if this is an internal call or an external call.

PSTN Numbering Plans

PSTN numbering plans are different for every country, but they share some common elements. Most PSTN numbering plans support the following types or classes of numbers:

Emergency services Many countries provide a single number to reach all emergency services. Others have separate numbers for police, fire, ambulance, and so on.

Information or directory services Most service providers have a number that can be used to reach directory services. In the United States, this is typically 411. Other services, such as current time or weather, might also be offered.

Local Local calls are calls within the same geographic area and typically can be dialed using fewer digits than long-distance calls. Local calls are typically free but might incur per-minute or per-call charges.

Mobile Most countries, with the notable exception of the United States, bill the caller for calls placed to mobile phones. This requires mobile phones to be assigned a distinct number range. In the United States, the owner of the mobile phone is billed for both incoming and outgoing calls, so mobile phones are assigned numbers from the same pool as traditional phones.

Toll or long distance Long-distance calls are directed to numbers outside the local area and commonly require an access code and area code in addition to the subscriber number. Historically, long-distance calls always incurred a per-minute toll. Many service providers now offer unlimited long-distance calls for a fixed fee.

Toll free Toll-free calls are free to the person who is making the call. The service provider bills the recipient.

Premium Premium calls are calls to businesses or services that invoke per-minute or per-call charges. Premium calls are classified distinctly from long-distance because they are frequently used for entertainment purposes, such as obtaining sports scores or using adult entertainment services. Most businesses block all outbound access to premium numbers, because these are rarely appropriate in a business environment.

International International calls are calls to other countries. They are placed using an access code followed by an International Telecommunications Union (ITU)-assigned country code and the full subscriber number, including any area or city codes.

The numbering plan for a country or region is typically complex. Over time, dramatic growth has caused most numbering plans to be revised, resulting in various exceptions or regional peculiarities. The following sections briefly introduce two common numbering plans for reference to show how various countries have implemented their numbering plans. For complete information on a particular country or regional numbering plan, please refer to the appropriate governing body.

As described in Chapter 5, "Circuit Options," the ITU Recommendation E.164 specifies how to build a numbering plan to allow interoperability between the numerous public networks. The E.164 segments a publicly assigned number into a country code (CC) and a national (significant) number, or N(S)N. The N(S)N is further segmented into a national destination code (NDC) and a subscriber number (SN). The DID numbers that a service provider assigns conform to the E.164 recommendation and are referred to as the E.164 number.

North American Numbering Plan

The North American Numbering Plan (NANP) is based on a ten-digit number assigned to each endpoint. This number is represented as XXX XXX-XXXX. The first three digits are the area code. The next three digits are the prefix and were originally used to route calls to the appropriate phone company switch. The last four digits are the subscriber number. Area codes are assigned geographically. Calls made within an area code typically use seven digitsthe prefix and the subscriber number. Some densely populated areas have more than one area code assigned.

Determining whether a call is considered a local or toll call can be problematic. In sparsely populated areas, calls within a single area code might be considered long distance. In a densely populated area, calls between area codes might be considered local. To confuse matters more, some densely populated areas use ten-digit dialing for all local calls, whereas others use both seven-digit and ten-digit local dialing.

UK National Numbering Plan

The UK National Numbering Plan (NNP) also uses an area code system. The area codes are known as city codes. Unlike the NANP, the UK NNP does not have a fixed length subscriber number. The number of digits assigned to a subscriber varies based on the city. Because of the variable length of the subscriber number, the area codes are also a variable length. Larger cities have a three-digit area code, whereas smaller cities might have a four-or five-digit area code. For example, Liverpool subscribers are assigned a seven-digit subscriber number and a three-digit area code of 151, whereas subscribers in Coventry are assigned a six-digit subscriber number and a four-digit area code of 2476. This allows most calls between city codes to use 11 digits.

Table 9-1 illustrates the structuring of the UK NNP.

Table 9-1. UK NNP

Area Code Prefix

Service Type

Example Format

00

International

00+countrycode+number

01

Area codes

Liverpool 0151 xxx xxxx

Leeds 0113 2xx xxxx

02

Area codes

London 020[378] xxx xxxx

Coventry 0247 6xx xxxx

03

Area codes (expansion)

04

Reserved

05

Corporate

BT broadband voice

055 xxxx xxxx

06

Reserved

07

Mobile/pager/personal

Mobile

07[789]xx xxxxxx

08

Freephone (also shared cost)

0800 xxx xxx

0800 xxx xxxx

0808 xxx xxxx

09

Premium

09xx xxx xxxx

Note

The area code prefix is commonly depicted with a leading 0, as shown in Table 9-1. The 0 is actually an access code indicating that the call is a national call, similar to the 1 used to indicate a long-distance call in the NANP.

Table 9-2 lists the format for the various classes of numbers for the NANP and the UK NNP.